NED Abstract

Copyright by American Astronomical Society.
Reproduced by permission
2012ApJS..203...17R
Angular Momentum and Galaxy Formation Revisited
Romanowsky, Aaron J.; Fall, S. Michael
Abstract. Motivated by a new wave of kinematical tracers in the outer
regions of early-type galaxies (ellipticals and lenticulars), we
re-examine the role of angular momentum in galaxies of all types. We
present new methods for quantifying the specific angular momentum j,
focusing mainly on the more challenging case of early-type galaxies, in
order to derive firm empirical relations between stellar j_sstarf_ and
mass M_sstarf_ (thus extending earlier work by Fall). We carry out
detailed analyses of eight galaxies with kinematical data extending as far
out as 10 effective radii, and find that data at two effective radii are
generally sufficient to estimate total j_sstarf_ reliably. Our results
contravene suggestions that ellipticals could harbor large reservoirs of
hidden j_sstarf_ in their outer regions owing to angular momentum
transport in major mergers. We then carry out a comprehensive analysis of
extended kinematic data from the literature for a sample of ~100 nearby
bright galaxies of all types, placing them on a diagram of j_sstarf_
versus M_sstarf_. The ellipticals and spirals form two parallel
j_sstarf_-M_sstarf_ tracks, with log-slopes of ~0.6, which for the spirals
are closely related to the Tully-Fisher relation, but for the ellipticals
derives from a remarkable conspiracy between masses, sizes, and rotation
velocities. The ellipticals contain less angular momentum on average than
spirals of equal mass, with the quantitative disparity depending on the
adopted K-band stellar mass-to-light ratios of the galaxies: it is a
factor of ~3-4 if mass-to-light ratio variations are neglected for
simplicity, and ~7 if they are included. We decompose the spirals into
disks and bulges and find that these subcomponents follow
j_sstarf_-M_sstarf_ trends similar to the overall ones for spirals and
ellipticals. The lenticulars have an intermediate trend, and we propose
that the morphological types of galaxies reflect disk and bulge
subcomponents that follow separate, fundamental j_sstarf_-M_sstarf_
scaling relations. This provides a physical motivation for characterizing
galaxies most basically with two parameters: mass and bulge-to-disk ratio.
Next, in an approach complementary to numerical simulations, we construct
idealized models of angular momentum content in a cosmological context,
using estimates of dark matter halo spin and mass from theoretical and
empirical studies. We find that the width of the halo spin distribution
cannot account for the differences between spiral and elliptical
j_sstarf_, but that the observations are reproduced well if these galaxies
simply retained different fractions of their initial j complement (~60%
and ~10%, respectively). We consider various physical mechanisms for the
simultaneous evolution of j_sstarf_ and M_sstarf_ (including outflows,
stripping, collapse bias, and merging), emphasizing that the vector sum of
all such processes must produce the observed j_sstarf_-M_sstarf_
relations. We suggest that a combination of early collapse and multiple
mergers (major or minor) may account naturally for the trend for
ellipticals. More generally, the observed variations in angular momentum
represent simple but fundamental constraints for any model of galaxy
formation.
Key words: galaxies: elliptical and lenticular, cD, galaxies: evolution,
galaxies: fundamental parameters, galaxies: kinematics and dynamics,
galaxies: spiral, galaxies: structure